In the comments to my post yesterday about Nanoarchaeum equitans, an ancient parasite, the discussion took an interesting turn.

Web Webster wrote: “So in a way, N. equitans is both ‘smarter’ in that it uses more of its total capabilities (versus humans and the old ‘10% of the brain thing’) and ‘more efficient’ in the way it works.”

To which Brent M. Krupp responded: “That ‘old “10% of the brain thing’ is complete and utter rubbish. Not a grain of truth to it, nor was there ever. Sorry to go off on this pet peeve of mine, but it’s unclear if you were serious in your reference to that myth.”

I can’t remember the first time I heard the claim that we only use 10% of our brain’s full potential. It always sounded dicey to me, maybe because I didn’t trust the people who pushed it. They’d always add that this “fact” meant that some New Age technique could liberate the other 90% of your brain power. As far as I could tell, they themselves had yet to liberate the first 10%.

Fortunately, we now live in an age when such myths can be torpedoed at DSL speed. The folks at Urban Legends offer a quick history of this bit of neurological misinformation. In a nutshell, in the 1930s neurologists figured out that only 10% of the human cortex becomes active during sensory stimulation or the motor control of the body. So the other 90% was referred to as “silent cortex.” This technical term doesn’t mean that that 90% is useless, only that it is silent in these particular tasks, like walking and smelling. In fact, these other regions become active in other kinds of thought–such as making decisions and recalling memories. But that didn’t stop the 10% figure from taking on a life of its own.

By coincidence, the 10% story has been on my mind again recently. Over the summer I came across a fascinating paper in Current Biology by Peter Lennie of New York University. Lennie takes a look at how much energy the cortex uses to think. First, he calculates the total amount of energy used by the human cortex, based on recent neuroimaging studies. Then he calculates how much energy a single neuron in the cortex uses when it generates an electric impulse. And finally, he uses these figures to estimate how many neurons in the cortex can be active at any one time. His estimate? Around one percent.

In a way, this finding is even more mind-blowing than the old 10% story. Now it seems that a full 99% of the human cortex is quiet at any time. This intense limitation can also help explain many features of the brain. It can account for the way the brain is arranged into specialized networks that can be rapidly adjusted as incoming information changes. It can account for the way neurons can jam-pack their signals with information. The cortex is a scarce resource, as Lennie’s paper makes clear, and evolution has found various ways to make the most of it.

It would be a mistake for anyone to turn Lennie’s results into a new urban legend about how we have yet to unlock 99% of our brain potential. For one thing, his work doesn’t mean that 99% of our cortex is permanently shut down–only that relatively few neurons are active at any moment. And getting the other 99%of the cortex to be active at the same time would be no easy task. Even at rest, our brains use 20% of the oxygen we take in, and we rely on an intricate mesh of blood vessels to cool off our brains as they use up all this energy. If you reached the full potential of your brain, it seems, you would burn it up in the process.